The use of air conditioning for cooling during warmer months has increased steadily in recent years. As a result, many utilities have experienced large power demands in the summer which impose a severe burden on their generating capacity. These peak loads require expensive peaking generators to meet the peak loads and require the utilities to maintain expensive generating capacity that is under-utilized during periods of lesser demand. Consequently, it is necessary to develop a means for shifting electricity consumption from peak demand periods to off-peak periods. One solution is the development of a heat pump thermal energy storage (TES) system that has the capability of removing heat from a storage material during off-peak periods and using the cooled storage material as a heat sink during peak demand periods.
Methods of incorporation of cool storage in heat pump and air conditioning systems have been developed, the most common being the use of water or ice as the storage medium. However, both have their disadvantages. Large volumetric storage capacities are required for cool storage systems which utilize the sensible heat capacity of water. In addition, water storage systems must typically operate over a large temperature range to be effective. In ice storage systems, the low temperatures that are required to freeze ice and the heat exchange penalty resulting from ice buildup on the heat exchanger coils reduce air conditioner performance and increase operating costs. In addition to water and ice, use of hydrated salts as a storage medium has also been investigated. Studies have also been performed to develop aqueous organic hydrates for use as a cool storage medium.
A continuing search for a better cool storage medium has led to the consideration of clathrates. Clathrates are distinguisned by having molecules of one type completely enclosed within the crystalline structure of molecules of another type. Thermal energy storage materials have been developed using clathrate-forming compounds dissolved in water. Gas hydrates are a subset of the class of the solid compounds called clathrates. For gas hydrates, a guest molecule is contained within a cage-like crystalline structure of the host water molecules (see FIG. 1). The guest molecules 1 stabilize the crystalline structure of the host by occupying the cavities formed within the crystalline water structure 3. For many guest molecules, this stabilization of the crystalline lattice has the effect of causing water to freeze at a temperature higher than is required under ordinary circumstances with only water molecules present. This is an advantage since the freezing point of water, 32.degree. F., is lower than that required for air conditioning purposes and causes a decrease in air conditioner performance.
Another advantage is that many common refrigerants, such as R-12 (CCl.sub.2 F.sub.2), form gas hydrates under appropriate conditions. Since the guest molecule of the gas hydrate can be the same as the refrigerant, the possibility exists to incorporate the cool storage medium directly into the heat pump cycle, thus allowing the use of direct contact heat exchange to charge the system and eliminating the need for an additional heat exchanger. This advantage will be further explained in the description of the invention. For these and other reasons gas hydrates offer advantages over water, ice, hydrated salts, organic hydrates and other clathrates as a cool storage medium.
Investigations using refrigerants as the guest component of a gas hydrate cool storage medium have been pursued both at Oak Ridge National Laboratory and Argonne National Laboratory. Continuing problems arose due to the immiscibility of refrigerants in water causing difficulty in both initial nucleation and completed reaction. Tests show that performance of the process of crystallization during gas hydrate formation varied considerably from ideal and prevented realization of the anticipated advantages of a gas hydrate system. It was believed this was due to the difficulties arising when the gases and liquids of the guest and host molecules did not mix in the melted noncrystalline state. This invention addresses this mixing problem and is a process for formation of a gas hydrate as a storage medium to be incorporated into a heat pump and air conditioning system.